Conventional magnetic resonance imaging (MRI) seeks to produce a single echo having a constant signal at a single point in time. Conventional MRI sequences may use a fixed set of flip angles to generate a signal at a single echo time (TE). However, application of multiple radio frequency (RF) pulses in a pulse sequence may produce multiple spin and stimulated echoes at times other than just the desired TE. Conventionally, these higher echo pathways are either refocused or spoiled to help the single desired signal reach a steady state.
Conventional magnetic resonance (MR) pulse sequences include a preparation phase, a waiting phase, and an acquisition phase that are configured to produce steady state signals from which images can be made. The preparation phase determines when a signal can be acquired and determines the properties of the acquired signal. For example, a first pulse sequence may be designed to produce a T1-weighted signal at a first echo time (TE) while a second pulse sequence may be designed to produce a T2-weighted signal at a second TE, where T1 is spin-lattice relaxation and T2 is spin-spin relaxation. These conventional pulse sequences are typically designed to provide qualitative results where data are acquired with various weightings or contrasts that highlight a particular parameter (e.g., T1 relaxation, T2 relaxation).
MR fingerprinting (MRF) takes a different approach. MRF sequences seek to generate unique signal evolutions using a combination of different acquisition parameters. MRF simultaneously generates quantitative maps by analyzing acquired spatially and temporally incoherent signals in light of a pre-calculated dictionary. Instead of working to produce a constant signal, MRF embraces signal dynamics by varying acquisition parameters. For example, flip angle and repetition time may be varied to generate unique signal evolutions for different tissue types.
MRF is described in Ma D, Gulani V, Seiberlich N, Liu K, Sunshine J, Duerk J, and Griswold M. Magnet Resonance Fingerprinting, Nature, 495:187-192 (2013). MRF is also described in U.S. patent application Ser. No. 13/051,144 filed Mar. 18, 2011 by Griswold et al., and in U.S. patent application Ser. No. 13/623,104 filed Sep. 19, 2012 by Griswold et al.